1. Field of the Invention
The present invention relates to an image forming apparatus and a nozzle restoring method, and more particularly, to an image forming apparatus which records images by discharging droplets of ink or the like onto a recording medium, and a nozzle restoring method for same.
2. Description of the Related Art
Conventionally, one known example of an image forming apparatus is an inkjet recording apparatus (inkjet printer) that has an inkjet head (ink ejection head) with an alignment of multiple nozzles and that forms an image on a recording medium by ejecting ink from the nozzles while moving the inkjet head and the recording medium relative to each other.
Various methods are known in conventional practice as ink ejection methods for such an inkjet recording apparatus. Known examples include a piezoelectric system wherein a vibration plate that constitutes part of a pressure chamber (ink chamber) is deformed by the deformation of a piezoelectric element (piezoelectric ceramics), the capacity of the pressure chamber is changed, ink is led into the pressure chamber from an ink supply channel during this increase in pressure chamber capacity, and the ink in the pressure chamber is ejected as droplets during the decrease in pressure chamber capacity. Further, known examples also include a thermal inkjet system wherein ink is heated to create air bubbles for ejecting the ink by the expansion energy when the air bubbles grow.
In an image forming apparatus having an ink discharge head such as an inkjet recording apparatus, ink is supplied to an ink discharge head via an ink supply channel from an ink tank which stores ink, and this ink is discharged by one of the various discharge methods described above. However, it is necessary that ink is discharged stably in such a manner that factors such as the ink discharge volume, the discharge velocity, the discharge direction, and the three-dimensional shape of the discharged ink, conform to uniform values at all times.
However, during printing, the nozzles of the ink discharge head are filled with ink at all times, in order that printing can be performed as soon as a printing instruction is issued. Therefore, the ink in the nozzles is exposed to the air, and the ink in nozzles which do not perform discharge for a long period of time dries, the viscosity of the ink increases, and nozzle blockages may occur. Increased viscosity at the ink meniscus in a nozzle can cause ink discharge failures. Furthermore, foreign matter, such as dust or air bubbles introduced into the ink supply channels may accumulate, thus blocking the ink supply and giving rise to discharge errors.
Therefore, various means have been devised in the related art in order to eliminate ink discharge errors and discharge failures of this kind. For instance, ink of increased viscosity at the meniscus region of a nozzle which may cause an ink discharge failure is subjected to a restoring process, such a purging (dummy discharge or preliminary discharge), or suction, whereby the ink of increased viscosity is expelled forcibly to the outside of the machine, at periodic intervals.
One known example discloses a system for an inkjet head having a plurality of printing nozzles connected directly to a common ink chamber and arranged in accordance with the pixel density, a discharge energy application device being provided inside each printing nozzle. Dummy nozzles, which are connected to the common ink chamber, have a smaller flow resistance than the printing nozzles, and do not contribute to printing, are provided with respect to the printing nozzles, in such a manner that dust, air bubbles, or the like, inside the inkjet head can be expelled effectively (see, for example, Japanese Patent Application Publication No. 6-270400).
Furthermore, an example is known in which a plurality of ink discharge ports of different diameters for discharging different quantities of ink are arranged in a discharge surface, and stable ink discharge is achieved by imparting an optimal shape to the liquid-attracting portion of the ink discharge surface in accordance with the amount of ink discharged from the respective ink discharge ports (see, for example, Japanese Patent Application Publication No. 8-39805).
Moreover, an example is known in which a plurality of discharge ports for discharging ink include large-diameter discharge ports and small-diameter discharge ports, the number of small-diameter discharge ports being greater than the number of large-diameter discharge ports. Ink flow channels connected to the large-diameter discharge ports and ink flow channels connected to the small-diameter discharge ports are arranged in a mixed fashion in line with ink supply ports, and a group of a plurality of small-diameter discharge ports is disposed between respective large-diameter discharge ports. In this way, it is possible to carry out a uniform restoring process of the large-diameter discharge ports and the small-diameter discharge ports (see, for example, Japanese Patent Application Publication No. 2003-127383).
Furthermore, an example is known in which the ambient temperature and humidity of a recording head is determined, and preliminary discharge, which is a discharge that does not contribute to recording, is performed on the basis of preliminary discharge drive conditions established in accordance with the determination results (see, Japanese Patent Application Publication No. 2000-190528). In this way, stable discharge can be achieved, even in an environment subject to variations in temperature and humidity.
However, in the related art, purging (preliminary discharge) for restoring nozzles which have produced a discharge error as described above is carried out on the basis of a timer or the number of droplet ejection operations performed, but since there is variation in the timing at which a discharge failure occurs, due to the environment (temperature and humidity) in the vicinity of the nozzles, the viscosity of the ink, and the like, then the timing for implementing purging is set so as to include a certain safety margin. Therefore, purging is carried out when the ink has not yet actually increased in viscosity and when purging is not actually necessary, and this means that the frequency of the restoring process is high and ink is consumed wastefully.
In Japanese Patent Application Publication No. 6-270400, the flow channel resistance of the dummy nozzles which expel dust, air bubbles, and the like, is less than the flow channel resistance of the printing nozzles. Therefore, during suction, a greater amount of ink is expelled from the dummy nozzles, and the printing nozzles may not be restored satisfactorily, in addition to which, a large amount of ink is consumed wastefully.
Moreover, in Japanese Patent Application Publication No. 8-39805, nozzles of different diameters are arranged in order to maintain recording density, and they are not used to control restoring processes. Therefore, the frequency of restoring processes remains high and ink is consumed wastefully.
In Japanese Patent Application Publication No. 2003-127383, by forming a plurality of small-diameter discharge ports between respective large-diameter discharge ports, a restoring process is carried out uniformly with respect to the small-diameter discharge ports as well as the large-diameter discharge ports. However, similarly to Japanese Patent Application Publication No. 8-39805, frequency of restoring processes remains high and ink is consumed wastefully.
Furthermore, in Japanese Patent Application Publication No. 2000-190528, the time until a preliminary discharge (purging) operation is previously specified and there may be cases where preliminary discharge is carried out when it is not actually necessary, thus causing wasteful consumption of ink. In Japanese Patent Application Publication No. 2000-190528, the temperature and humidity of the recording head are determined, and the timing of preliminary discharge is established in accordance with these determination results. However, there also exist other environmental factors, such as the wind force that acts on the meniscus surface when the recording head is moved back and forth over the recording medium, the heat radiated from the recording medium, and the like, and hence the timing of preliminary discharge cannot be specified on the basis of the ambient temperature and humidity of the recording head alone.